1. Field of the Invention
The present invention relates to a superjunction semiconductor device.
2. Description of Related Art
It is well-known that IGBTs (insulated gate bipolar transistors) are used as switching devices in inverter circuits or power circuits provided in various types of home appliances such as refrigerators, air-conditioners, and laundry machines; energy-related systems such as solar power generating systems and wind power generating systems; and vehicles such as electric vehicles (EV) and hybrid electric vehicles (HEV), for example.
An IGBT disclosed in Patent Document 1 includes: an n− drift layer; a p-base layer formed on the n− drift layer; an n+ emitter layer formed on a portion of the surface of the p-base layer; a trench formed so as to penetrate the n+ emitter layer; a gate electrode formed in the trench across a gate insulating film; an n-buffer layer formed on the bottom of the wafer; a p-collector layer formed further towards the bottom of the wafer than the n-buffer layer; an emitter electrode formed on the top of the wafer; and a collector electrode formed on the bottom of the wafer.
Patent Document 1: Japanese Patent No. 5036327
In the set shown as an example above, there has been demand for more energy savings for all embedded applications in order to reduce the impact on the environment. The IGBT switching device, however, differs from a MOSFET in that the IGBT is a bipolar device, and thus, an ON voltage greater than or equal to the VF (forward voltage) to the current is necessary. As a result, if using IGBTs in a motor driving circuit, for example, the efficiency of the set using this motor driving circuit will be low in low voltage ranges.
On the other hand, the MOSFET, which is a unipolar device, can form a set with excellent efficiency in low voltage ranges as compared to the IGBT if used in the above-mentioned set, and therefore, is used instead of the IGBT. In general, however, the chip size of the MOSFET must be made larger in order to be compatible with both low voltage ranges and high voltage ranges, which leads to an increase in cost.
The MOSFETs that are used as switching devices in inverter circuits and power circuits are largely separated into planar types and superjunction types. Planar MOSFETs include a drain layer, an n-type base layer arranged on this drain layer, a p-type base layer formed on the surface of the n-type base layer, and an n+ drain layer and n+ source layer formed on the surface of the p-type base layer with a gap therebetween, for example. The gate electrode is arranged so as to face the surface of the p-type base layer between the n+ source/drain layer across the gate insulating film.
As disclosed in Patent Document 2, superjunction MOSFETs include a p-type columnar region that extends from the p-type base layer towards the drain layer, in addition to the configuration of the planar MOSFET described above, for example. This structure enables a reduction in on-resistance and improves switching speed.
Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2012-142330
One problem with superjunction MOSFETs is the hard recovery of the parasitic diode. Hard recovery means that the change in reverse recovery current (dir/dt) is fast. In superjunction MOSFETs, a depletion layer spreads from both the p-type base layer and the p-type columnar region when the parasitic diode is turned off. In particular, the depletion layer that spreads from the p-type columnar region quickly bonds with the depletion layer that spreads from another adjacent p-type columnar region and quickly reaches the drain layer directly below. Therefore, the current changes rapidly, and blocking of the reverse recovery current also occurs at a high speed. In response to this, the reverse recovery current waveforms exhibit an oscillation (ringing) with steep changes and a large amplitude. Such reverse recovery characteristics (hard recovery characteristics) cause a large amount of noise, and could cause the controller supplying control signals to the MOSFETs to malfunction, for example. In particular, in an inverter circuit that drives an inductive load such as in an electric motor, the parasitic diode turns ON and OFF; therefore, the hard recovery characteristics when this parasitic diode is turned off poses a problem. According to Patent Document 2 mentioned above, the reverse recovery characteristics are improved by heavy particle irradiation from the rear surface of the n-type drain layer with heavy particles such as protons, 3He++, and 4He++, but this does not improve the hard recovery characteristics.